Nowadays thrombus lysis is mainly effected by means of chemical agents, such as tissue plasminogen activator, streptokinase or urokinase which are administered by the IV route. Current results indicate that in the presence of an occlusive clot, these are effective in only about 60 per cent of the cases.
The biological use of focused high-intensity acoustic energy was first studied by Lynn et al., in 1942. They demonstrated that focused acoustic energy can produce deep localized damage at the focal point in various biological tissues in vitro and in vivo, with a minimal effect on the surface and no effect on the intervening tissue.
About 30 years later utilization of focused acoustic energy re-emerged in the biomedical field. Experimental investigation in the use of focused shock waves has led to non-surgical treatment of upper urinary tract calculi. Extensive experience with shock wave lithotripsy has found the method to be safe and highly effective. About 80-90% of the patients with "surgical" calculi were successfully treated, only 7% of the treated patients required intervention to resolve residual problems after shock wave lithotripsy.
The mechanism of shock wave lithotripsy is believed to be associated with the acoustic impedance mismatch between the target calcific calculus (stone) and the surrounding (non-calcific) soft tissue. When the shock waves reach the target, the acoustic impedance mismatch gives rise to pressure changes of magnitudes sufficient for shattering the target calculus. Generally very short, intense pulses generated by a spark gap in a liquid, are used.
The inventor started work in 1985, on harnessing acoustic energy in order to utilize it in interventional cardiology. At the first stage he developed an ultrasound ablation catheter (U.S. Pat. No. 5,163,421). This catheter was a breakthrough in the treatment of coronary artery disease, but it is an invasive procedure which is of course a drawback.
This development was initially met with skepticism by experts in this field as there persisted a general belief that it would not be possible to attain a selective ablation of a thrombus, occluded in an artery, so that the artery would not be damaged. We demonstrated that the arterial wall is very resistant to high energy ultrasound energy. Thrombus is very sensitive to ablation by high energy ultrasound.
Thus, Acoustic energy in interventional cardiology is a "smart energy" which enables the performance of interventional procedures to open up "thrombus-rich" occlusions in arteries with a wide margin of safety. Currently, the ultrasound angioplasty catheter is in the final stages of in vivo evaluation of a coronary prototype.
In 1990 the inventor started to explore the feasibility of non-invasive acoustic ablation of thrombi. A report was published in the American Journal of Cardiology (Rosenschein U, Guberinich D, Yakubov SJ, Bach DS, Abrams GD, Sonda PL, EJ Topol, "Shock Wave Thrombus Ablation: A new method for non-invasive mechanical Thrombolysis". Am J. Cardiol 1992; 70;1358-1361). The objective was to evaluate for the first time the ability of non-invasive acoustic energy to selectively ablate a thrombus embedded within an arterial segment. The purpose of these experiments was to show that there can be effected a selective ablation of the thrombus, essentially without damage to the arterial wall by the acoustic waves. It seems that the focused acoustic wave generates cavitation which depolymerizes fibrin, thus disintegrating the thrombus.
In spite of the fact that such acoustic energy is successfully used with calculi for more than 20 years, no one considered it feasible to use acoustic energy for selective ablation of soft tissue, specifically of thrombi in blood vessels.